The invention relates to analog circuit design, and in particular embodiments to low voltage integrated circuits in which current mirroring is employed.
In analog integrated circuitry there is often a requirement to provide a precise ratio of currents based on a reference current. Providing such currents is. commonly accomplished using current mirrors.
Modern integrated circuits typically operate with reduced supply voltages, in order to conserve energy and to accommodate low voltage digital circuits. As the components within integrated circuits continue to shrink, circuit breakdown voltages typically decrease and supply voltages decrease accordingly. Because of the lower supply voltages within modern integrated circuits, power supplies used for current mirrors and other analog circuitry may be constrained to operate with reduced supply voltages. Accordingly, the voltage available for the functioning of current mirrors is decreased and performance may suffer. Because of decreasing supply voltages, circuit parameters may have an increasing effect on the current provided by current mirrors. Accordingly, there is a need within the art for improved biasing techniques for use with current mirrors.
Embodiments of the present invention attempt to maintain the proper current ratio between a reference current and the output current of the current mirrors. Embodiments of the current invention attempt to maintain the proper current ratio between the reference current and output current of current mirrors through methods applied to the reference side of the current mirror. This method of compensation using the reference side of the current mirror may be more effective than attempting to increase the current in the output sides of the current mirror, especially in those cases in which the supply voltage of the output current side is low. If most of the supply voltage is dropped across the load, of the output side of the current mirror, no voltage headroom may be left to perform current regulation necessary to maintain the proper ratio between reference and output currents.
Embodiments of the present invention may include such methods as matching the voltage across the output device in the reference side of the current mirror to the voltage drop in the output device of the output side of the current mirror. Embodiments of the present invention may also include various measures to insure that the internal impedance of the reference side is proportional to the impedance of the output side of the current mirror in such a ratio as to maintain the proper current ration between the reference current and the output current.